1. Field of the Invention
The invention relates to ceramic substrates for thin and thick layer electronic circuits and to the process for producing the same whereby a ceramic mass containing alumina, an organic bonding agent, a plasticizing agent, an emulsifier and water is transformed by extrusion into a cohesive, flat strip, which is then dried to a "green" state and divided or cut into desired components, which are then fired or sintered into solid ceramic substrates.
2. Prior Art
Because ceramic materials have outstanding insulating properties, they comprise a preferred material in the production of substrates for electronic circuits. Depending upon the intended utility, particular requirements are made as to the flatness and surface quality of such ceramic substrates. Thus, the surface roughness of a ceramic substrate to be used with thin film electronic circuits must be less than about 0.2 .mu.m, since otherwise it is not possible to apply a continuous circuit component film in the thicknesses typically required for thin film component elements. On the other hand, where the ceramic substrates are to be used with thick film electronic circuits, the requirements regarding substrate surface quality are less strict since in thick film techniques, thicker circuit component layers are typically used for conductor paths, resistors, capacitors and the like. Further, an increased substrate surface may even be desirable with thick film electronic circuits in order to achieve improved adherence of the circuit component layers onto the substrate.
In order to achieve a high surface quality, ceramic substrates for use with thin film electronic circuits are generally produced in accordance with the so-called "foil casting" process as disclosed, for example, in U.S. Pat. No. 2,966,719. Generally, this foil casting process comprises preparing a slip or readily flowable or castable ceramic mass by mixing finely distributed ceramic materials, such as alumina, a volatile organic solvent, a wetting agent and an organic bonding agent which is soluble in the solvent utilized. A softener or lubricant which is compatible with the bonding agent may, optionally, also be added. This slip is then degassified and cast or poured onto a very smooth carrier consisting of, for example, glass. Then the solvent is removed, as by drying whereupon the cast slip assumes a tough, flexible leather-like "green" state which may be shaped as a strip or the like. This "green" strip is cut or divided, for example by punching, into desired substrate shape components, which are then transformed or cured into solid ceramic bodies by sintering or firing. Following the firing process, the surface roughness of such solid ceramic substrates amounts to less than about 0.2 .mu.m. A disadvantage of this foil casting process comprises the relatively expensive preparation of the slip since a constant viscosity monitoring and expensive vacuum degassification process are required. Further, the initial drying process must be carried out relatively slowly in order to avoid cracks and the like and as a result of such slow drying, the quantity of ceramic substrates which can be produced per unit time is considerably limited. In addition, undesirable thickness variations tend to occur in such finished ceramic substrates due to fluctuations of viscosity in the slip.
A variation of the above-described foil casting process is disclosed in U.S. Pat. No. 4,009,938 (owned by the instant assignee) wherein the "green" dried shaped substrate components, prior to sintering, are compressed between two highly polished, plane-parallel surfaces. This relatively simple measure improves the flatness and surface quality of the ultimate ceramic substrates.
A process for producing ceramic substrates which is considerably simpler than the afore-described foil casting process and which is considerably cheaper is the so-called "extrusion process", which is disclosed, for example, in U.S. Pat. No. 2,952,877. Generally, this process comprises producing an extrudable ceramic mass comprised of, for example, alumina and bonding and plasticizing additives in a milling means to obtain a substantially homogeneous mixture, feeding such mixture to an extrusion means and extruding the mass through a slot-like nozzle within the extrusion means so as to form an endless flat strip, which is then dried relatively rapidly to the "green" state and stamped or divided into shaped articles which are then fired or sintered to form rigid bodies. The ceramic substrates produced in accordance with this extrusion process have a surface roughness (i.e. an average peak-to-valley height) of more than 0.5 .mu.m so that as a practical matter said substrates can, at best be, only utilized with thick film circuits.